The present invention relates to a method of manufacturing spherical alumina by the oil-drop method. More particularly, it relates to a method of manufacturing sperhical alumina using gibbsite as the material for preparing the alumina hydrogel that is a precursor of said spherical alumina.
The oil-drop method as taught in U.S. Pat. No. 2,620,314 has been widely utilized in the preparation of spherical alumina employed as a catalyst or as a catalyst carrier. Briefly, this method comprises commingling an alumina hydrosol with a gelling agent which hydrolyzes at an elevated temperature, dispersing the resulting mixture as droplets in a suspending medium thereby forming hydrogel particles, ageing the thus obtained hydrogel particles, washing with water, drying and calcining.
Generally speaking, it is preferable in the above-mentioned oil-drop method that the alumina hydrosol, which is a precursor of the spherical alumina, should be free from impurities. This is because the impurities contained in the alumina hydrosol reside per se in the spherical alumina and are liable to act as catalyst poisons. Due to this, the high purity alumina hydrosol for use in the oil-drop method has usually been prepared by means of either a method of digesting metallic aluminum with an aqueous hydrochloric acid solution and/or an aqueous aluminum chloride solution or a method of electrolyzing an aqueous aluminum chloride solution in an electrolytic cell provided with a porous partition wall between the anode and cathode when an impurity-containing aluminum chloride is employed as the starting material. However, as the former must use relatively expensive metallic aluminum and the latter must use a special electrolytic cell, it may safely be said that the prior art is at any rate destined to be unable to produce the high purity alumina hydrosol cheaply and consequently the spherical alumina obtained therefrom is expensive.
As is generally known, gibbsite is the principal ingredient of bauxite, and is available more cheaply than high purity metallic aluminum or aluminum chloride. However, in view of the fact that about 0.2 wt. % to 0.5 wt. % of sodium, which acts as a catalyst poison, is contained in gibbsite in the form of Na.sub.2 O, gibbsite, even though cheap, could not be used as the material for preparing the alumina hydrosol unless said sodium is removed by some means or other.
We have discovered that gibbsite can be utilized as part of the material for preparing the alumina hydrosol which is a precursor of the spherical alumina and that the sodium contained in the gibbsite resides per se in the hydrogel particles obtained by the oil-drop method, but said sodium can be removed very easily by washing the hydrogel particles with water.